Abstract

Most particle motions on Earth's surface are fundamentally stochastic and often occur under rarefied transport conditions. Every particle makes a unique path along the bed, similar but distinct from the paths of all other particles in motion, until it loses enough kinetic energy to become disentrained. The details of a particle's motion are determined by the amount of energy added and extracted during each moment of travel. Thus, particle motions physically reflect the complex energy dynamics at play and are a building block of morphodynamic theory. A full appreciation of this energy balance is needed to properly describe the motion of particles and associated disentrainment under different transport conditions. Often multidimensional behaviors occur during transport as both a result of and influence on these particle scale energy dynamics. One such phenomenon is that of particle-scale random walking during transport which results in diffusion over short timescales in both the downstream and transverse directions. We have adopted the Galton board as the fundamental conceptual model on which to create a mechanistic yet probabilistic formulation of particle diffusion. Here we provide a data set of two-dimensional particle travel distances supplemented with high-speed videos of particle-surface collisions collected during laboratory experiments to characterize the influence of shedding fluid vortices and angularity on collisional distances and two-dimensional travel for particles at low Reynolds numbers. Such a description is consistent with diffusion from the top-down and may be distinct from the bottom-up, or surface roughness, controlled random walking that other studies have explored. Preliminary analysis shows that spherical particles experience jiggling motions resulting in transverse displacement in the absence of surface roughness and this behavior is further exaggerated for particles of natural angularity. We hope to clarify the influence of the particle Reynolds number in top-down and bottom-up spreading.